Prolonged Exposure (PE) is a form of individual psychotherapy based on the Foa and Kozak emotional processing theory [4], which posits that phobic disorders and PTSD involve pathological fear structures that are activated when information represented in the structures is encountered. Emotional processing theory purports that fear memories include information about stimuli, responses, and meaning and fear structures are composed of harmless stimuli that have been associated with danger and are reflected in the belief that the world is a dangerous place. This belief then manifests itself in cognitive and behavioral avoidance strategies that limit exposure to potentially corrective information that could be incorporated into and alter the fear structure. As escape and avoidance from feared situations are intrinsically (albeit, temporarily) rewarding, phobic disorders can perpetuate without treatment.

Consequently, several theorists have proposed that conditioning processes are involved in the etiology and maintenance of anxiety disorders. These theorists invoke Mowrer’s two-factor theory [5], which posits that both Pavlovian and instrumental conditioning are involved in the acquisition of fear and avoidance behavior. Successful treatment requires emotional processing of the fear structures in order to modify their pathological elements so that the stimuli no longer invoke fear, and any method capable of activating the fear structure and modifying it would be predicted to improve symptoms of anxiety . Imaginal PE entails engaging mentally with the fear structure through repeatedly revisiting the feared or traumatic event in a safe environment. The proposed mechanisms for symptom reduction involve activation and emotional processing, extinction/habituation of the anxiety, cognitive reprocessing of pathogenic meanings, the learning of new responses to previously feared stimuli, and ultimately an integration of corrective non-pathological information into the fear structure [6, 7].

When PE is used for PTSD, the approach typically involves the graded and repeated imaginal reliving and narrative recounting of the traumatic event by the patient within the therapeutic setting. Although PE relies primarily on sensory memory and imagination, the exposure process is not passive. Patients are asked to verbally recount their trauma experience in the first person with their eyes closed, as if it were happening again with as much attention to sensory detail as they can. Using clinical judgment, the therapist might prompt the patient with questions about their experience or provide encouraging remarks as deemed necessary to facilitate the recounting of the trauma narrative. This approach is believed to provide a low-threat context where the patient can begin to confront and therapeutically process trauma-relevant memories and emotions as well as decondition the learning cycle of the disorder via a habituation/extinction process.

VR can be seen as an advanced form of human–computer interaction [8] that allows the user to “interact” with computers and digital content in a more natural or sophisticated fashion relative to what is afforded by standard mouse and keyboard input devices. Immersive VR can be produced by combining computers, head-mounted displays (HMDs), body-tracking sensors, specialized interface devices, and real-time graphics to immerse a participant in a computer-generated simulated world that changes in a natural/intuitive way with head and body motion.

One common configuration employs a combination of an HMD and head tracking system that allows delivery of real-time images and sounds of a simulated virtual scene rendered in relation to user movements that corresponds to what the individual would see and hear if the scene were real. Thus, an engaged virtual experience creates the illusion of being immersed “in” a virtual space within which the user can interact.

The use of VR to address psychological disorders began in the mid-1990s with its use as a tool to deliver exposure therapy targeting anxiety disorders , primarily for specific phobias (e.g., heights, flying, spiders, and enclosed spaces). At the time, VR was seen to be capable of immersing an individual in a digital 3D graphic rendering of a feared environment, within which activation and modification of the fear structure was possible. Exposure was the first psychological treatment to use VR in part due to the intuitive match between what the technology could deliver and the theoretical requirement of gradual exposure to systematically expose users to progressively more challenging stimuli as needed to activate the fear structure. Moreover, even during the early days of VR, this was not so technically challenging to achieve. Virtual environments (VEs) could be created that required only simple navigation within a simulation where users were presented with scenarios that resembled key elements of the targeted fear structure that could be made progressively more provocative (views from tall buildings, aircraft interiors, spiders in kitchens, etc.). And even with the limited graphic realism available at the time, phobic patients were observed to be “primed” to suspend disbelief and react emotionally to virtual content that represented what they feared.

In general, the phenomenon that users of VR could become immersed in VEs provided a potentially powerful tool for activating relevant fears in the treatment of specific phobias in the service of therapeutic exposure. A growing body of controlled studies targeting specific phobias has emerged since 1995; two meta-analyses of the early literature concurred with the finding that VRE is an efficacious therapeutic approach [9, 10], and a more recent meta-analysis and a systematic review of this literature have expanded on the findings in this area [11, 12]. These initial reviews support the notion that VR is an effective tool for fostering therapeutic exposure within an evidence-based cognitive behavioral therapy (CBT) protocol to treat these types of anxiety disorders.

In the late 1990s, researchers began to test the use of VRE for the treatment of PTSD by systematically immersing users in simulations of trauma-relevant environments. While the efficacy of imaginal PE for PTSD has been established in multiple studies with diverse trauma populations [13–15], it is reported that some patients are unwilling or unable to effectively visualize the traumatic event [1]. This is a crucial concern since avoidance of cues and reminders of the trauma is one of the cardinal symptoms of the DSM 5 diagnosis of PTSD [16]. In fact, research on this aspect of PTSD treatment suggests that the inability to emotionally engage (in imagination) is a predictor for negative treatment outcomes [17].

Similar to its use in treating specific phobias, the rationale for using VR as a tool to deliver PE for PTSD is clear and compelling. Clients can be immersed in simulations of trauma-relevant environments in which the emotional intensity of the scenes can be precisely controlled by the clinician to customize the pace and relevance of the exposure for the individual patient. In this fashion, VRE offers a way to circumvent the natural avoidance tendency by directly delivering multisensory and context-relevant scenes and cues that aid in the retrieval, confrontation, and processing of traumatic experiences. Within a VR environment, the hidden world of the patient’s imagination is not exclusively relied upon.

The first effort to apply VRE for PTSD began in 1997 when researchers at Georgia Tech and Emory University began testing the Virtual Vietnam VR scenario with Vietnam veterans diagnosed with PTSD [18]. This occurred over 20 years after the end of the Vietnam War. During those intervening years, in spite of valiant efforts to develop and apply traditional psychotherapeutic and pharmacological treatment approaches to PTSD, the progression of the disorder in some veterans significantly impacted their psychological well-being, functional abilities, and quality of life, as well as that of their families and friends. This initial effort yielded encouraging results in a case study of a 50-year-old, male Vietnam veteran meeting DSM IV-R criteria for PTSD [19]. Results indicated posttreatment improvement on all measures of PTSD and maintenance of these gains at a 6-month follow-up, with a 34 % decrease in clinician-rated symptoms of PTSD and a 45 % decrease in self-reported symptoms of PTSD.

This case study was followed by an open clinical trial with Vietnam veterans [18]. In this study, 16 male veterans with PTSD were exposed to two HMD-delivered virtual environments, a virtual clearing surrounded by jungle scenery and a virtual Huey helicopter, in which the therapist controlled various visual and auditory effects (e.g., rockets, explosions, day/night, and shouting). After an average of 13 exposure therapy sessions over 5–7 weeks, there was a significant reduction in PTSD and related symptoms. For more information, see the 9-minute Virtual Vietnam Documentary video at: http://​www.​youtube.​com/​watch?​v=​C_​2ZkvAMih8.

Initial positive results were reported in a case study by Difede et al. [1] for PTSD related to the terrorist attack on the WTC using VRE with a patient who had failed to improve with traditional imaginal exposure therapy. The authors reported a 90 % reduction in PTSD symptoms as measured by the “gold standard” clinician-administered PTSD scale (CAPS), and an 83 % reduction in depressive symptomatology as measured by the Beck Depression Inventory [20]. This research group later reported positive results from a wait-list-controlled study using the same WTC VR application [21]. The VR group demonstrated statistically and clinically significant decreases on the CAPS relative to both pretreatment and to the wait-list control group with a between-groups posttreatment effect size of 1.54. Seven of the ten people in the VR group no longer carried the diagnosis of PTSD, while all of the wait-list controls retained the diagnosis following the waiting period and treatment gains were maintained at 6-month follow-up. Also noteworthy was the finding that five of the ten VR patients had previously participated in imaginal PE with no clinical benefit, and showed a 25–50 % improvement following VRET. Such initial results were encouraging and suggested that VR may be a useful component within a comprehensive treatment approach for persons with terrorist attack-related PTSD. For more information, see the Virtual World Trade Center video at: http://​www.​youtube.​com/​watch?​v=​XAR9QDwBILc.

Initial clinical tests of the Virtual Iraq/Afghanistan PTSD VRE system have also produced promising results. Three early case studies reported positive results using this system [22–24]. In the first open clinical trial [25], analyses of 20 active duty treatment completers (19 male, 1 female, mean age = 28, age range: 21–51) produced positive clinical outcomes with statistically and clinically meaningful reductions in PTSD, anxiety, and depression symptoms resulting in 16 clients no longer meeting PTSD criteria on the PCL-M [26]. These improvements were also maintained at 3-month posttreatment follow-up. Another open clinical trial with active duty soldiers (n = 24) produced significant pre-/postreductions in PCL-M scores and a large treatment effect size (Cohen’s d = 1.17) [27]. After an average of 7 sessions, 45 % of those treated no longer screened positive for PTSD and 62 % had reliably improved. In a small preliminary quasi-randomized controlled trial [28], 7 of 10 participants with PTSD showed a 30 % or greater improvement with VR, while only 1 of 9 participants in a “treatment as usual” group showed similar improvement. The results are limited by small size, lack of blinding, a single therapist, and comparison to a set relatively uncontrolled usual care condition, but it did add to the incremental evidence suggesting VR to be a safe and effective treatment for combat-related PTSD .

Positive results from uncontrolled open trials are difficult to generalize and caution is necessary such that excessive claims are not made on the basis of these early results. However, the overall trend of these positive findings (in the absence of any reports of negative findings) is encouraging for the view that VRE is safe and may be an effective approach for delivering an evidence-based treatment (PE) for PTSD . Three randomized controlled trials (RCTs) are currently ongoing using the Virtual Iraq/Afghanistan system with SMs and veteran populations. One RCT is focusing on comparisons of treatment efficacy between VRET and imaginal PE [29], and another is testing VRET compared with VRET + a supplemental care approach [30]. One other RCT is investigating the additive value of supplementing VRET and PE with a cognitive enhancer called D-cycloserine (DCS) [31, 32].

DCS, an N-methyl-d-aspartate partial agonist, has been shown to facilitate extinction learning in laboratory animals when infused bilaterally within the amygdala (“fight or flight” conditioning center in the brain) prior to extinction training. Recent evidence of both VRET and DCS effectiveness has been reported by Difede et al. [33] in a clinical trial with WTC PTSD patients. In a double-blinded controlled comparison between VRET + DCS and VRET + Placebo, both groups had clinically meaningful and statistically significant positive outcomes, with the DCS group achieving statistically greater gains at 6-month follow-up. Significant funding support for these RCTs underscore the interest that the DOD/VA has in exploring this innovative approach for delivering PE using VR.